1. Field of the Invention
The present invention relates to high SBS threshold optical fibers.
2. Technical Background
Stimulated Brillouin Scattering (SBS) is a dominant nonlinear penalty in many optical transmission systems. In many systems, it is desirable to transmit large optical power through optical fibers, while maintaining high signal to noise ratio (SNR). However, as the power of the incident optical signal launched into an optical fiber increases, it may exceed a certain threshold power (SBS threshold) and part of the signal power will then be reflected back due to SBS. Thus, due to SBS, a large amount of the signal power can be lost due to reflection back toward the transmitter. In addition, the scattering process increases the noise level at the signal wavelength. The combination of decrease in signal power and increase in the noise both lower the SNR and lead to performance degradation.
At finite temperatures, thermal excitations in glasses occur similarly to that of phonons in crystals, and the interaction of these vibrational modes with low intensity signal light produces spontaneous Brillouin scattering. An intense optical field generates pressure or sound waves through electrostriction due to the beating of intense incident and spontaneous reflected light, giving rise to pressure or acoustic waves. The change in pressure causes material density to change, thereby resulting in refractive index fluctuations. The net result is that an intense electrical field component of the optical wave generates pressure or sound (acoustic) waves which cause material density fluctuations. The acoustic wave changes the refractive index and enhances the reflected light amplitude through Bragg diffraction. Above the SBS threshold of an optical fiber, the number of stimulated photons is very high, resulting in a strong reflected field which limits the optical power that is transmitted, and which reduces the SNR.
U.S. Pat. Nos. 6,856,740 and 6,687,440 disclose the use of acoustic wave anti-guiding to reduce SBS. This is achieved by an optical fiber core that is doped such that the longitudinal acoustic velocity of the fiber core is higher than that of the cladding. (That is, “the effective index of refraction” for the acoustic wave is lower than that of the cladding.) However, our analysis showed that this technique will have limited utility because acoustic cladding modes become prevalent in the absence of core modes. The acoustic cladding modes then couple into the core, creating SBS and establishing the SBS threshold. Furthermore, one technique for achieving acoustic wave anti-guiding within the fiber core utilizes special coatings. The low damage threshold of such coatings precludes the use of such fiber in high optical power applications.
US patent application No 2004/009617 and U.S. Pat. No. 6,587,623 also disclose a similar SBS reduction technique. These references are directed to SBS reduction via reduction of the acoustic core modes, by allowing acoustic modes to propagate in the fiber cladding. Again, this approach fails to consider acoustic cladding modes which couple into the fiber core and then become confined inside the core, where they overlap with optical modes and play a major role in the SBS threshold. Furthermore, these references focus on shear velocity when longitudinal acoustic velocity is known to be the dominant parameter in SBS.
U.S. Pat. No. 6,542,683 discloses that the SBS effect is mitigated by an optical fiber with a core with both radially nonuniform viscosity and non-uniform CTE provided by alternating different layers of glass, via modifying dopants such as phosphorous and fluorine. The patent teaches that the thickness of the alternating layers should be less than 0.5 μm, and the co-dopants between two adjacent layers are different by at least one composition.
The paper by Y. Koyamada et al. (J. of Lightwave Technology, vol. 22, pp. 631-639, 2004) discloses a method of suppressing acoustic modes propagating in the fiber core by making the longitudinal acoustic velocity in the core higher than in the cladding. This is achieved by doping the fiber cladding with Fluorine, thereby decreasing the cladding's refractive index while lowering acoustic velocity within the cladding. The amount of Ge in the core was decreased to provide the appropriate delta, relative to the cladding. The paper teaches for the fiber with a 1 μm radius (Ge doped) core with the very high refractive index delta (3.7% delta relative to the cladding) and Fluorine doped cladding, minimum SBS (high SBS threshold) occurs when the longitudinal acoustic velocity difference between the core and cladding is 0.03.
The paper by P. D. Dragic et al. (CLEO'2005, paper CThZ3, Baltimore, Md., May 22-27, 2005) discloses a fiber design with a ring shaped acoustic field guiding layer that surrounds the core. However, this design did not account for the acoustic cladding modes, which can coupled into the core region, creating SBS and establishing the SBS threshold. The experimental results reported in the paper show that the fiber attenuation is much higher than the standard single mode fiber, which is not desirable for fiber laser applications.